Copper plating bath and process



Oct. 17, 1950 w. A. PRoELL COPPER PLATING BATH AND PROCESS Filed Sept. 24.A 1947 OW Db 3EME SRU um GM Patentcd Det. 17, 1950 COPPER PLATING BATH AND PROCESS Wayne A. Proell, Chicago, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application September 24, 1947, Serial No. 775,833

6 Claims.

This invention relates to novel baths suitable for the electrodeposition oi.' copper. More particularly, it relates to copper electroplating baths wherein the principal or substantially the sole anion employed for the transport of electrical current is derived from an alkanesulfonic acid. This invention also relates to novel processes for the electrodeposition of copper.

Copper has heretofore been plated commercially principally from acid sulfate and from cyanide baths. Both types of commercial copper electrodepositing baths have undesirable limitations. Thus, the above-mentioned commercial baths are usually operable at relatively low current densities. The acid sulfate bath is usually not operated at current densities above about 150 amperes per square foot and copper cyanide baths are operated at far lower densities, usually not exceeding about 60 amperes per square foot. The copper acid sulfate bat-h requires the incorporation of various addition agents in order that it may be used to deposit copper as a lustrous film. Although the copper cyanide baths deposit copper having a ne grain, their use is attended by the above-mentioned disadvantage of operation at low current densities,` by the poor protective quality of the copper plate which is deposited and by the inherent danger to health of a cyanide bath because of the accidental possibility of its contamination with acidic materials which would result in the production of hydrogen cyanide.

Accordingly, there is commercial demand for a copper electrodepositing bath which can be operated over a wide range of current densities, including high current densities of 500 amperes or `more per square foot to produce adherent mat or lustrous copper deposits. There is also commercial demand for electroplating baths in which the copper concentration can be varied over a wide rangaand which may be operated over a wide range lof operating conditions. There is, moreover, commercial necessity for copper plating baths which can be operated to produce smooth,

. lustrous electrodeposits at high speed without the necessity of incorpotating addition agents in the baths.

It is an object of my invention to provide novel copper electrodepositing baths which are operable over a wide range oi conditions, the essential component of said baths being a copper alkanesulfonate of an alkanesulfonic acid having 1 to 5 carbon atoms, inclusive, in the alkyl group. Another object of my invention is to provide electroplating baths containing copper salts of alkanesulfonic acids having 1 to 5 carbon atoms,

inclusive, in the alkyl group, which baths can be mat electrodeposits of copper. An additional object of my invention is to provide processes for the employment of the novel copper alkanesulfonate electroplating baths of my invention. These and other objects will become apparent from the ensuing description of my invention.

Alkanesulfonic acids suitable for employment in the copper electroplating baths of this invention may be prepared by a variety of methods. Especially suitable methods comprise the catalytic oxidation of mercaptans or aliphatic suldes having the formula RiSnRz wherein R1 and R2 are alkyl groups having 1 to 5 carbon atonns, inclusive, and n is a positive integer between 1 and 6, preferably 2. Air or oxygen may be used as the oxidizing agent and various nitrogen oxides are employed as catalysts, the oxidation being aivfooted at temperatures below about 300 F.

These catalytic oxidation processes are described and claimed in applications for United States Letters Patent S. N. 571,023, led by W. A. Proel] and B. H. Shoemaker on January 1, 1945, now

, form. The aqueous copper alkanesulfonates of alkanesulfonic acids having between 1 and about 5 carbon atoms, inclusive, in the molecule are extremely stable and well adapted to electrolysis.

The copper alkanesulfonate baths of this invention-are extremely versatile bothas to their composition and as to the conditions of electroplating under which they can be employed. Thus, these baths may be operated over a broad temperature range, usually at temperatures between about F. and about 200 F. The copper concentration in the copper alkanesulfonate baths can vary between about 10 grams ofdissolved copper per liter of solutionto grams or even more (depending upon the solubility of the particular copper alkanesulfonate at the particular electroplating temperature) per liter of solution.

Solubilifies were determined in Water and dilute alkanesulfonic acids of copper alkanesulfonates having the formula Cu(SO3R)z.2H;O having the molecular weight of 329, prepared by copper carbonate neutralization of a mixture of C1C4 alkanesulfonio acids having the molecular weight of 113. The copper alkanesulfonates ex-v hibited virtually identical solubilities in water, 1.0% aqueous Ci-C4 alkanesulionic acid `mix-- I Wei ht Percent Temp., Cu( OaRJLZHIO F. in Saturated Solution It is ordinarily desirable to maintain free alkanesulfonic acid in the copper plating baths, i. e. alkanesulfonic acid in excess of lrhe stoichiometric amount in combination with copper. Ordinarily the free alkanesulfonic acid concentration is maintained at a value between about and about 50 grams per liter of solution. although it is usually desirable, for reasons which will hereinafter be pointed out, to maintain between about 5 and about 10 grams per liter of 'free alkanesulfonic acid. Depending upon the composition of the bath, the temperature and other variables, as will hereinafter be detailed, the copper alkanesulfonate baths of this invention may be operated over an extremely wide range of current densities varying from about to about 1500 amperes per square foot.

In accordance with this invention, copper was plated from aqueous solutions of copper salts of mixtures of alkanesulfonic acids, such mixtures comprising predominantly methane, ethaneand propane-sulfonic acids. The results hereinafter described were obtained from such a mixture of alkanesulfonic acids having, before dilution. the specific gravity of 1.35 and normality of 12.08 (as determined by titration with normal alkali solution to a pH of 4.5 with a glass electrode), containing 92.7 weight per cent oi' the mixed alkanesulfonic acids having an average molecular weight of 113,` 3.2,weight per cent H2804 and 4.1 weight per cent of water. A l weight per cent aqueous solution of this mixture of alkanesulfonic acids has a pH ofV about 1.15.

In preparing copper alkanesulfonate baths, the requisite weight of alkanesulfonic acid was diluted with water to approximately threefourths the required volume. The requisite weight of chemically pure basic cupric carbonate was added, as a. slurry, slowly with stirringto the acid solution.

The weight of cupric carbonate was calculated as follows:

4 had passed through the solution vAny sludge or precipitate formed was filtered oif. The copper and free acid was checked by analysis and corrected, if necessary. The bath was then ready for plating.

If it was necessary to add copper carbonate in the bath, the stoichiometric quality of acid was added also, to maintain the free-acid value.

It', after a plating campaign, it was found necessary to add alkanesulfonic acid. it did not appear necessary to "dummy" the bath again. However, the bath was given an activated carbon treatment, especially if a substantial amount of fresh'alkanesulfonic acid were added.

(g./l. copper) X (liters of bath) X (1.03) wt. cupric per cent copper in cupric carbonate carbonate The extra 3 per cent copper, indicated in the above calculation (1.03), is necessary to replace the copper precipitated during the dissolution of the cupric carbonate in the acid.

After all the cupric carbonate was dissolved, the bath was diluted to volume with distilled water. Then 20 g./l.l of a powdered diatomaceous earth which had been calcined with a fusible alkali metal salt (Hyflo Super Cel filter Y The baths were easily controlled by periodic chemical analyses for copper and free alkanesulfonic acid. Since the effect of free alkanesulfonic acid concentration was more critical than the copper content, the free acid was determined more frequently.

Performance studies were carried out in nine i-liter baths. They contained 25, 50, and g./l. of copper in solution, and for each copper concentration baths containing l0, 25,` and 50 g./1. of free alkanesulfonic acid were prepared, by the procedure above described. Hereinafter the copper plating baths will be frequently designated by two numbers separated by a hyphen. the first of which is the grams of copper in solution per liter of bath and the second of which is the grams of free alkanesulfonic acid per liter of solution. Thus bath 25-10 contains 25 g./l. of copper in solution and 10 8./1. of free alkanesulfonic acid per liter.

Each plating cell had two electrolytic copper anodes, 2 inches by 3 inches submerged area, one on each side of the cathode. The cathode was a soft brass panel. 1 inch by 3 inches submerged area. 'I'he panels were cleaned as follows:

' '(a) Degreased in trichloroethylene;

(b) Soaked for 2 minutes at 180 F. in a hot alkaline cleaner containing:

(c) Dipped for 1.5 minutes at F. in ammonium chloride-hydrochloric acid containing:

1 50 g./l. oi' ammonium chloride and i2 cc./l. of concentrated hydrochloric acid.

" It should be understood that other cleaning procedures known to the art may be used before plating.

` A fine, reddish-brown sludge is formed on the anodes during plating. If the -bath is not agitated, and if plates of the order of 0.001 inch are made, it may not be necessary to bag the anodes. However, solution agitation dislodges the sludge and may give rise to nodular plates, especially if the plates are heavy. Also, the sludge falls into the bath whenever the anode is moved or lifted from the bath. Therefore, it is recommended'that the anodes be bagged. Duck canvas bags were employed about the -anodes but any acid-resisting, closely-woven material should be satisfactory. The amount of sludge formed appears to decrease as the anode current density is increased. The amount of' sludge wasdess than that formed in a copper sulfate bath and slightly more than in a" iluoborate bath.' For nuoborate baths. the sludge formation increasesV s with increase in current density, instead of decreasing asin the case of the alkanesulfonate baths.

PlatesV approximately 0.001 inch thick were made in each bath at increasing current densities, starting from 20 amp/sq. ft., at 90, 120, and 150 F. The current density, at which the first signs of burning were observed, was taken as the limiting current density...` Thelimiting current density was confirmed by making a plate at a higher current density and noting the lncrease in burned area.

The cathode was given a slow reciprocal movement with the plane of the panel being parallel to the plane of the anodes and to the direction of motion. The movement consisted of a 2 inch stroke at 32 strokes per minute.

For bath 25-10, the pH limits are 1.5 to 1.2 for a free-acid content between 5 and 10 g./l. Similarly, for bath 50-10, the pH limits are 1.4 to 1.0,v

and for bath 100-10, the pH range is 1.3-1.0. For operating control, titration with standard alkali solution to pH of 4.5 (glass electrode) is preferred for free acid determination.

The accompanying figure graphically presents the results obtained in 204 tests of copper plating from copper alkanesulfonate baths of the compositions andunder the conditions set forth in the figure. The 204 tests represent a broad range of bath compositions and operating variables.

Probably the most significant single fact discernible from the accompanying ligure is that it was possible to obtain commercially usable copper electrodeposits from extremely variant bath compositions under extremely variant operating conditions. It is believed that the accompanying gure is substantially self-explanatory. suppemented as it is by the description of bath compounding and operating procedures presented herein.

Compositions of copper alkanesulfonate plating baths and suitable operating conditions are shown in Table 1. 'Ihe bath number has two components referring directly to g./i. of copper and maximum g./l. of free-acid.

Table 1.Bath compositions and operating conditions suitable for copper alkanesulfonate baths *Agitatiizgn by work-rod movement oi 2 inch stroke and 32 strokes er minu p Limiting current density ls deiined for the present purpose es the maximum amperage per square loot which gives satisfactory 0.00l-inch plates.

Plating in bath 25-10 is suitably effected at the nominal average current density of 60 amp/sq. ft.. which is compatible with the capacity of most present-day plating installations. Bath temperature can be maintained at 120 F. or 150 F. This bath is characterized by a very good factor of safety in current density range for irregularly shaped objects where the current density at some areas may very considerbecome finer grained at current densities aboveA amp/sq. ft. The plates made at 60 amp/sq. ft. have a hard crystal structure and require a fast, hard wheel for buiiing. 'I'hese plates have a. textured `appearance after electrobufllng However, the plates produced at 100 amp/sq. ft. readily buff to a smootlfsurface by both methods.

The plates from bath 50-10 are smooth and fine grained `at the nominal average` current density of 150. amp/sq. ft. They are easily bufted by customary mechanical methods and by electrolytic method. The latter reveals a somewhat crystalline appearance. v 4 l Y Successful operation of bath 100-10 requires more rigid control than baths 25-10 and 50-10. Both temperature and current density are important factors in maintaining a lustrousplate with bath 100-10. For cases where high ductility is not important. `such as on die castings, current densities between 60 and 500 amp/sq ft. can be used at 90 F. Thus, at 90 F. a wide range of current density is permissible, although the ductility of the plate is not as good asthat obtained at higher temperatures. For good ductility, 4temperatures between about 120 F. and 150 F. should be used. 'At 120 F., the current density range is limited to about to about 600 amp/sq. ft. to maintain a lustrous copper plate, and at 150 F'. current i densities abovr 500 amp/sq. ft. are employed. Hence, fc lustrous copper plates at about 150` to 20h amp/sq. ft., bath 100-10 must be used at temperatures between about 90 and 120 F., depending upon the ductility desired.

The ductility mentioned above, refers to the ease of bending the plate sharply through If cracks appear, the plateis not called highly ductile. The plates exhibiting less ductility are not brittle. but after being bent, they occasionally have fine hair-line cracks. This phenomenon is not general; some plates crack` and some do'not, hence there is only a tendency to show hair line cracks. When these plates are bent through a larger radius (1 inch or greater), no cracks are visible. All the plates thatshow no cracks when bent sharply through 180 are called highly ductile. and, at any set of conditions, all plates, not Just a few, must exhibit that characteristic before the condition is reported as giving ductile plates.

The lustrous copper plates show excellent ease of mechanical hurling and electrobuing. No graininess is apparent after bung. In addition, bright nickel plated directly upon a lustrous copper electro-deposit has a good color. This property is a. distinct advantage for a simple acid copper bath, since a bufilng step maybe eliminated with signicant ccst saving.

At this point more detailed consideration will be given to the effects or some of the electro- Vplating variables.

`tility is caused by an increase in free acid. De-

creasing the free alkanesulfonicacid concentration below 5 g./l. increases the limiting current 'densityto as high as 1400 amp/sq. ft., but

asado/is bending. Baths 25-10 and 50-10 are not as sensi- Y tive to free alkanesulfonic acid concentration as bath 100-10, but if these baths are operated near the limiting current density, the free acid must be maintained between the limits given. 100-10 is extremely sensitive to free alkanesulionic acid concentration, insofar as limiting current density is concerned. Except for a slight decrease in ductility, however. good electroplates of copper are obtainable from alkanesulfc-nate baths containing as muchas 50 g./l. free alkanesulfonic acid. However, the limiting current density is reduced from 1200 to 400,amp./sq. ft. by increasing the free alkanesulfonic acid concentration from to 50 g./l. in a bath containing 100 g./l. of copper in solution.

Increasing the copper content of the alkanesulfonate bath increases the limiting current density, and conversely a decrease of the copper concentration decreases the limiting current density of the bath.

Temperatures of about 90 F. to above 150 F. are preferred in plating with the baths of this invention. To c-btain the most ductile copper electrodeposits, temperatures between about 120 F. and about 150 F. are preferred. Below about 120 F., the plates which are deposited are less ductile than those produced in the preferred temperature range and heavy plates sometimes crack on bending. Increasing the plating temperature to a value within the preferred range increases the ductility of the resultant plates 'and the limiting current density. The temperature of 150 F. is especially recommended for copper alkanesulfc-nate plating baths. Above about 150 F., water evaporation and acid volatilization or change may become suiliciently aggravated to constitute a. disadvantage.

Increasing the current density at which electroplating is eiected results in copper plates exhibiting liner grain, and, as the limiting current density of the baths is approached, the copper plates which are produced become lustrous or ysemi-bright, even though no addition agents are incorporated in the bath. Increasing the current density appears to decrease the ductility slightly. However, plates made at or near the limiting current density can be bent 180 around a 1A," radius without cracking. The plates are line-grained, mat, smooth, and satiny at current densities just below the current densities producing the lustrous plates.

The grain size of the plates produced from copper alkanesulfonate baths is much ner than those which copper sulfate and copper iiuoborate baths yield, and approaches the fineness exhibited by the plate obtained from copper cyanide baths. However, after a thickness of approximately 0.003 inchto 0.005 is exceeded, the plates from the copper sulfonate baths containing no added addition agents begin to show coarser grains.

A striking feature of the plates produced by copper alkanesulfonate baths is the absence of the columnar structure normally associated with acidic copper baths. The copper plates produced from the present baths have a crystal structure independent of the crystal structure of the metal upon which the plate is formed. Plates having this characteristic are advantageous for covering surface defects in the -metal upon which the copper electrodeposit is formed.

Bath

' throwing power than other acidic copper plating 8 High cathode current densities can be obtained in the alkanesulfonate baths of this invention at low agitation rates. Bathconcentrations both for metal and acid inthe baths of this invention l are-lower for the current densities attainable thanv those for copper viluobo-ratebaths. The present baths were observed to exhibitasgood or better Certain addition agents are of Value in the copper alkanesulfonate baths of my invention` 'I'he addition to the alkanesulfonate baths of between about 5 and about 50 g./1. each of tartaric acid and boric acid can improve the ductility and adherence of the copper electrodeposits when alkanesulfonic acids containing organic impuri` ties are employed in preparing the baths.

This application for Letters Patent is a, continuation in part of my previous application, Serial No. 602,406, filed on June 29, 1945.

I claim:

1. A copper electroplating bath comprising an aqueous solution of a copper alkanesulfonate having between 1 and 5 carbon atoms, inclusive, in the alkyl group, said solution containing at least about 100 grams of dissolved copper per liter and at least about 5, but not more than 25 grams of uncombined alkanesulfonic acid having between 1 and 5 carbon atoms, inclusive, in the alkyl group,per liter.

2. A copper electroplating bath comprising an aqueous solution of a. copper alkanesulfonate having between 1 and 5 carbon atoms, inclusive, in the alkyl group, said solution containing at least about 100 grams of dissolved copper per liter and at least 5, but not more than about 10 grams of uncombined alkanesulfonic acid having between 1 and 5 carbon atoms, inclusive, in the alkyl group, per liter.

A3. A process for the electrodeposition of copper which comprises electrolyzing an aqueous solution of a copper alkanesulfonate having between 1 and 5 carbon atoms, inclusive, in the alkyl group, said solution containing at least about 100 grams of dissolved copper per liter and at least 5, but not more than about 25 grams of uncombined alkanesulfonic acid having between 1 and 5 carbon atoms, inclusive, in the alkyl group, per liter at a temperature between about 120 F. and about 150 F. and a current density of at least 500 amperes per square foot.

4. A process for the electrodeposition of copper which comprises electrolyding an aqueous solution of a copper alkanesulfo-nate having between 1 and 5 carbon atoms, inclusive, in the alkyl group, said solution containing at least about 100 grams of dissolved copper per liter and at least 5, but not more than about 10 grams of uncombined alkanesulionic acid having between 1 and 5 carbon atoms, inclusive, in the alkyl group, per liter at a temperature between about F. and about 150 F. and a current density of at least 500 amperes per square foot.

5. A process for the electrodeposition of copper which comprises electrolyzing an aqueous solution of a copper alkanesulfonate having between 1 and 5 carbo-n atoms, inclusive, in the alkyl group, said solution containing at least about grams of dissolved copper per liter and at least 5 but not more than about 10 grams per liter of uncombined alkanesulfonic acid having between l and 5 carbon atoms, inclusive, in the alkyl group, at a temperature between about F. and about F. and a current density of 500 to 800 amperes per square foot.

2,525,948 9 10 6. A process for the electrodeposition of cop- REFERENCES CITED per which comprises electrolyzing an aqueous soe uo i r f n lution of acopper alkanesulfonate having between mglf gans nte. ere ces are of record in the 1 and 5 carbon atoms, inclusive, in the alkyl group, said solution containing at least about 10o 5 UNITED STATES PATENTS grams of dissolved copper per liter and at least 5 Number Name Date bul-l not more than about 25 grams per liter of 2,111,575 Stack Marl 22l 1933 uncombined alkanesulfonic acid having between 2,294,053 Stack Aug 25, 1942' 1 and 5 carbon atoms, inclusive, in the alkyl group, at a temperature of about 150 F. and a 10 OTHER REFERENCES current density of 500 to 800 amperes per square Blum et al., Principles of Electroplating and foot. Electroforming. McGraw-Hil1 Book Co., New

WAYNE A. PROELL. York, 1930, P5868 89, 90, and `93. 

1. A COPPER ELECTROPLATING BATH COMPRISING AN AQUEOUS SOLUTION OF A COPPER ALKANESULFONATE HAVING BETWEEN 1 AND 5 CARBON ATOMS, INCLUSIVE, IN THE ALKYL GROUP, SAID SOLUTION CONTAINING AT LEAST ABOUT 100 GRAMS OF DISSOLVED COPPER PER LITER AND AT LEAST ABOUT 5, NOT MORE THAN 25 GRAMS OF UNCOMBINED ALKANESULFONIC ACID HAVING BETWEEN 1 AND 5 CARBON ATOMS, INCLUSIVE, IN THE ALKYL GROUP, PER LITER. 